1. Field of Invention
The present invention relates to a technique for accurately estimating vehicle speed to prevent erroneous control of a vehicle control device such as an anti-skid control device (ABS), traction control device (TRC) or rear wheel steering device (4WS).
2. Description of Related Art
For a vehicle control device that executes a vehicle control operation based on the vehicle speed, the validity of the wheel speeds, which are used to calculate the vehicle speed, is critical. In particular, if the vehicle is traveling at a low speed, the validity of detection values may be inaccurate because the outputs from wheel speed sensors are below the detectable speed level. Thus, it is necessary to accurately determine the validity of the wheel speeds using the lowest calculated wheel speed value, in other words, a wheel speed validity threshold.
For example, Japanese Patent Application Publication No. 7-257352 (JP-A-7-257352) describes a vehicle control device that selects the higher of the lowest calculated wheel speed and a permission speed, which is a threshold speed for permitting execution of the vehicle control, and compares the selected value and the vehicle body speed to determine whether to execute a control operation.
However, the vehicle control device described in JP-A-7-257352 may be unable to set a target vehicle speed of the vehicle properly and cause speed hunting depending on the lowest calculated wheel speed when executing a constant speed traveling control operation at a low speed of 0 to 5 km/h as disclosed in Japanese Patent Application Publication No. 2004-90679 (JP-A-2004-90679). Also, the actual vehicle speed may deviate greatly from the target vehicle speed when a constant speed traveling is performed at a low speed.